Specifying only the day in the timer string destabilizes snapd

[Mark M Evans]

apps:
  destroy-snapd:
   daemon: oneshot
   command: bin/do-nothing
   timer: sat # <------ Totally borks snapd

Attempting to install a snap with the above timer string will cause snapd to quickly run out of memory. The install will fail but snapd will now be stuck in a cycle of consuming all memory and restarting. Rebooting does not resolve the problem.

# Tested Build Environment

## MacOS
 * Catalina (10.15.3 (19D76))
 * snapcraft (3.9.1)
 * multipass (1.1.0+mac)

## Tested Runtime Environment

 Ubuntu Core 16

    caracalla 16.04-1.38 52 stable canonical✓ gadget
    caracalla-kernel 4.4.0-176.206 127 stable canonical✓ kernel
    core 16-2.43.3 8689 stable canonical✓ core

Example project: https://github.com/markmevans/day-only-timer-bug

[Maciej Borzecki (maciek-borzecki)]

This is likely a bug in snapd. We cover plenty of edge cases for the timer schedules, but obviously missed the simplest one for the service times. In this particular case, the systemd timer snapd wrote did not have any `OnCalendar` entries. Systemd 245 rejects such unit, but older versions might have done something weird.

Opened a PR with a fix to snapd: https://github.com/snapcore/snapd/pull/8333

[Zygmunt Krynicki (zyga)]

This was fixed in snapd 2.44.2, marking as such.